Method of constructing a flexible sand in the soft ground

ABSTRACT

A sand drain comprising a tubular protector 5 to 25 cm in diameter prepared by weaving monofilaments in the lattice form to provide a mesh area of 2.0 to 3.0 mm2 and the sand filled into the protector to form a sand pillar, which, after constructed in soft ground, can be rendered flexible so as to match any displacement of strata of the soft ground, thereby preventing breakage of the sand pillar from breakage.

liitie States atet 1 'lerashima et al.

[ Jan. 14, 1975 METHOD OF CONSTRUCTING A FLEXIBLE SAND DRAIN Inventors: Osamu Terashima, Tokyo; 111110 Okabayashi, Yamato, both of Japan Chiyoda Chemical Engineering & Construction Co. Ltd., Yokohama-shi, Japan Filed: July 23, 1973 Appl. No.: 381,587

Assignee:

Foreign Application Priority Data May 22. 1973 Japan 4856378 US. Cl 61/11, 61/35, 61/535,

61/63 Int. Cl E02b 11/00, EOZd 5/40 Field of Search ..61/11, 13,63, 53.5, 53.52, 61/5364, 53.66, 35

[56] References Cited UNITED STATES PATENTS 1,449,032 3/1923 Blumenthal 61/53.66

3,187,513 6/1965 Guild 61/535 3,396,541 8/1968 Lamberton 3,396,546 8/1968 Pleuger 61/5366 3,406,524 10/1968 Blenkarn 61/535 Primary Examiner-Jacob Shapiro Attorney, Agent, or Firm-Holman & Stern [57] ABSTRACT A sand drain comprising a tubular protector 5 to 25 cm in diameter prepared by weaving monofilaments in the lattice form to provide a mesh area of 2.0 to 3.0 mm and the sand filled into the protector to form a sand pillar, which, after constructed in soft ground, can be rendered flexible so as to match any displacement of strata of the soft ground, thereby preventing breakage of the sand pillar from breakage,

3 Claims, 9 Drawing Figures PATENTED JAN I 4 I975 SHEET 2 BF 3 FIG.

FIG.6

PEG. 7

PATENTEMM'QS 3.859.798

SHEET 30F 3 METHOD OF CONSTRUCTING A FLEXIBLE SAND DRAIN BACKGROUND OF THE INVENTION This invention relates to a method of constructing a sand drain and more particularly to a flexible sand drain embedded in soft ground.

Where a sand drain is to be provided in soft ground, mere formation of a sand pillar by drilling the ground and filling sand thereinto renders the sand pillar incapable of withstanding a shearing force originating with the displacement of the soft ground, with the result that the sand pillar is bent and finally pinched off, namely, has the upper and lower portions separated from each other, often failing to attain the proper function of a sand drain. To avoid these difficulties, there has hitherto been used a sand drain constructed by filling sand into a protector made of such cloth or paper as is not easily breakable. In the U5. Pat. No. 3,396,541 is set forth a typical sand drain using a protector made of woven fabric.

The sand drains proposed to date consist of a large diameter sand pillar having a broad shearing area, and a protector prepared from material of great tensile strength. However, a sand pillar constructed by the prior art has been readily broken by a shearing force. Moreover, the soft ground in which a sand drain is provided exerts an extremely strong shearing force to the sand drain when it is additionally subjected to a load. Therefore, the conventional sand drain has a limited capacity to withstand the shearing force.

The prior art protector of woven fabric has such small meshes as to allow the influx of water into the sand filling in the protector, but prevents the sand therein from being drawn out of the protector. Accordingly, the flow of underground water into a sand drain causes clay, entrained with the water, to obstruct the passages of the fine meshed wall of the protector, consequently forming a water-impermeable film on the protector. As the result, the protector has its fine meshes clogged, obstructing the passage of water into a sand drain, and reducing the drainage of water for which the sand drain is primarily intended. Further, it takes a great deal of time to construct large diameter sand drains in a required number one by one. Though it might be considered advisable to build many large sand drains at the same time by a single pile driver in order to eliminate the above-mentioned difficulties, yet an ordinary pile driver practically fails to carry out such work. Therefore, any process known to date has been unable to shorten the period of constructing a plurality of sand drains in the ground.

Moreover, the construction of a large diameter sand drain consumes a great deal of sand, and a protector used with such sand drain is subject to rigid requirements in respect of quality and mechanical strength, and unavoidably becomes expensive, resulting in economic disadvantage.

SUMMARY OF THE INVENTION It is accordingly an object of this invention to provide a method of constructing a flexible sand drain for soft ground which maintains a water-draining capacity even when in use for a long time, is readily rendered sufficiently flexible to match any displacement of soft ground, and is consequently only minimally subject to shear breakage. A flexible sand drain is provided comprising a sand pillar and tubular protector for supporting the sand, said protector being prepared from plain woven fabric constituted by warps and wefts of antiweathering, chemically noncorrosive monofilaments of great tensile strength interlaced such that the intrafilament meshes of the fabric are large enough to allow the passage through the protector of such a small amount of the sand of the sand pillar as prevents reduction in the required diameter of the sand drain.

The protector should preferably be formed by arranging the warps lengthwise of the protector and the wefts circumferentially of the protector at right angles to the warps, causing the protector as a whole to present a lattice formation.

For fabrication of a protector, it is desired to super- 1 pose two elongate sheets of woven fabric and join the sheets along the lines slightly in sidle of both lengthwise edges to provide lug portions all along said edges, thereby preventing the protector from being twisted when it is inserted into a casing. The intrafilament meshes are chosen to have an area of 2.0 to 3.0 mm A suitable protector is the one prepared from two sheets of monofilament woven fabric having a thickness of 300 to 500 deniers, which has a diameter of 5 to 25 cm and, as measured per 5 cm width when the protector is flattened, indicates a lengthwise tensile strength of more than 100 kg, a crosswise tensile strength of more than kg and an extensibility of less than 5 percent.

The method of this invention for constructing sand drains in soft ground consists in arranging in a regular polygonal form on the soft ground a plurality of casings each provided with a freely operable sealing cover on the top and having a sufficiently large inner diameter to allow the insertion of a protector, and driving the casings into the soft ground at the same time by a vibrohammer mounted on a pile driver. Thereafter, the protectors are inserted into the casings with the upper ends of the protectors connected to the outlets of a sand hopper provided on the pile driver. Sand is filled into the protector by the vibration of the vibro-hammer. After the casing is closed with the sealing cover at the upper end of the casing, compressed air is introduced into the casing so as to prevent a sand drain from being lifted. While being shaken by the vibro-hammer, the casing alone is pulled out of the ground, thereby enabling the protector now filled with sand to be firmly set in soft ground without being drawn out together with the casing. It is possible to insert the protector into the casing and flll the protector with sand before the casing is driven into the ground.

Where a sand drain is constructed in soft ground under water, provision of a rod having a length equal to or larger than the water depth between the vibrohammer and a casing enables the casing to be driven into the soft ground under water so as to bring the upper end of the casing near the surface of said ground.

BRIEF DESCRIPTION OF THE DRAWINGS This invention will be described. by way of example with reference to the accompanying drawings in which:

FIG. 1 is an oblique view of a tubular protector used with a flexible sand drain according to this invention which is constructed in soft ground;

FIG. 2 is a fractional enlarged view of the protector of FIG. 1;

FIG. 3 is a cross sectional view of the protector of FIG. 1;

FIG. 4 schematically illustrates the method of this invention for building a sand drain in soft ground;

FIG. 5 is a front elevation of the main part of a pile driver used in constructing the sand drain of FIG. 4;

FIG. 6 is a cross sectional view on line 66 of FIG. 5;

FIG. 7 is a cross sectional view on line 7-7 of FIG.

FIG. 8 schematically shows the condition of the pile driver of FIG. 5 where casings are suspended therefrom in a state ready to be driven into soft ground; and

FIG. 9 schematically indicates the condition of the pile driver where it is ready to drive casings into underwater soft ground.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Throughout the figures, the same parts are denoted by the same numerals.

A sand drain 10 according to this invention comprises a tubular protector l1 and a sand pillar 12 formed of sand 13 filled thereinto (FIG. 4).

The protector 11 is formed in the following manner. Water-repellent, anti-weathering, chemically noncorrosive monofilaments of great tensile strength are roughly interlaced into the lattice form to provide plain woven fabric 15, whose enlarged view is given in FIG. 2. Referring to FIG. 2, the vertically extending monofilaments 14 are warps 16 and the horizontally extending monofilaments 14 are wefts 17. As later described, the intrafilament meshes 18 of the fabric 15, namely, the openings defined by the adjacent warps l6 and adjacent wefts 17 are each chosen to have an area of 2.0 to 3.0 mm to attain the object of this invention. To provide intrafilament meshes 18 having the abovementioned size, it is advised to use monofilaments 14, or warps l6 and wefts 17, both having a thickness of 300 to 500 deniers. Experiments show that, as measured per 5 cm width when flattened, the protector 11 should have a lengthwise tensile strength along the warps 16 of more than I00 kg and a crosswise tensile strength along the wefts 17 of more than 75 kg, and an extensibility of less than 5 percent. Polyethylene, for example, may be used as a suitable raw material of monofilaments which meets the above-mentioned requirements.

The protector I1 is formed, as shown in FIG. 1, by superposing two narrow strips of the above-mentioned fabric 5 obtained by cutting along the warps l6 and joining both strips along the lines 19 about 20 mm inside of both lengthwise (hereinafter referred to as the joints") by thermal fusion, sewing or interlacing each other carried out along the warps 16. Thus, the protector 11 comprises a main body 20 defined between the joints l9 and two pairs of lengthwise elongate lugs 21 disposed adjacent to the main portion 20. The lugs 21 positioned outside of the joints 19 may be fused or interlaced with each other for each pair. In the protector 11, the warps 16 are arranged lengthwise and the wefts 17 are interlaced therewith at right angles, minimizing the extensibility of the former in the lengthwise direction and that of the latter in the crosswise direction.

The distance between the joints 19 of the fabric 15, namely the width of the main body 20 is chosen to range between about 8 cm and about 39 cm depending on the object for which the subject sand drain is used in order that, where the protector 11 is expanded by being filled with sand, it may indicate a round cross section, as shown in FIG. 3, 5 to 25 cm in diameter D.

The protector 11 is inserted, as shown in FIG. 4, into a casing 22 previously driven vertically into soft ground and then filled with sand 13. When the casing 22 alone is pulled out, a sand drain 10 is constructed.

A pile driver 23 shown in FIGS. 5 to 8 is provided with a vibro-hammer 24. Arms 25 horizontally extending from the side wall of the vibro-hammer 24 engage upright guiding members 26 to permit the vertical movement of the vibro-hammer 24. To the bottom of the vibro-hammer 24 is attached a power transmitting block 27, to the bottom end of which a mount 28 is fixed to receive the casings 22. Case-fixing means 28a (FIG. 6) are provided on the fixing plate 28 at the apices S of a polygon whose center lies on the axis 0 of the block 27. Though the polygon is a square in FIGS. 5 to 8, any other polygon may obviously be used. The block 27 is fitted with a sand hopper 29 bored with a plurality of outlets 29a which, when the casings 22 are fitted to the fixing plate 28, are brought right above the upper ends 22a of the casings 22.

The guiding members 26 are fitted at the lower end 26a with a horizontal guide plate 31 bored with openings 30 (FIG. 7) through which the casings 22 are conducted to the apices of the aforesaid polygon.

Referring to FIG. 8, the vibro-hammer 24 can be lifted to any desired height with the aid of a wire 34 by a drive means 33 mounted on the base 32 of the pile driver 23.

There will now be described the process of simultaneously constructing sand drains in soft ground. First, the vibro-hammer 24 of the pile driver 23 is lifted to such height that when the upper ends 22a of the casings 22 are attached to the fixing plate 28, the casings 22 can be set fully upright on the soft ground 35. Under this condition, the casings 22 are positioned, as previously described, at the apices of a polygon.

Upon completion of the above-mentioned preliminary operation, the wire 34 is loosened to actuate the vibro-hammer 24. The casings 22 are simultaneously driven by the vibration of the vibro-hammer 24 into the soft ground 35 until the lower ends 22b of the casings 22 are brought down to the prescribed depth. Thus, a plurality of casings 22 can be efficiently driven at a time, as easily understood, into the soft ground 35 by a single pile driver 23. FIG. 4 illustrates the sand drainconstructing method of this invention. Though, for briefness of description, only one casing 22 is indicated in FIGS. 4(A) to 4(C), a plurality of casings 22 can, of course, be constructed at the same time by a single pile driver shown in FIGS. 5 to 8. FIG. 4(A) shows the condition in which the driving of the casing 22 was completed. To the upper end 22a of the casing 22 is hinged a freely operable sealing cover 36. With the sealing cover 36 kept open as shown in FIG. 4(B), the protector 11 is inserted into the casing 22 to the prescribed depth. To facilitate said insertion, it is advised to attach a hook 37 to the lower end portion 11b of the protector I1 and suspend a weight 38 from the hook 37. The weight 38 consisting, for example, of sand wrapped in a piece of woven fabric enables the protector II to be easily brought down to the bottom of the casing 22. Otherwise, it is possible to fasten or close the lower end portion 11b of the protector 11 after placing some amount of sand in the lower endportion 11b of the protector 11 in advance. Even this process can attain the object all the same. To effect the easy insertion of the protector 11 into the casing 22, a clearance of 2 to 5 mm is allowed between the inner wall of the casing 22 and the outer wall of the protector 11 depending on the diameter of the latter. Accordingly, the casing 22 which receives the protector I1 of5 to 25 cm in diameter, is chosen to have an inner diameter of about 5.4 to 26 cm.

The two pairs oflugs 21 formed all along both lengthwise edges ofthe protector 11 (FIGS. 1 to 3) enable the protector 11 to be inserted into the casing 22 without being twisted, facilitating the subsequent filling of sand into the protector 11.

Upon completion of the insertion of the protector l 1, its upper end 11a is connected to the corresponding outlet 29a of the sand hopper 29 so firmly as to prevent the upper end 11a from falling off the outlet 29a. Thereafter, the vibro-hammer 24 is made to vibrate to let fall sand 13 from the hopper 29 into the protector 11. As the sand 13 is gradually filled into the protector 11 a small portion of the sand 13 flows out, as shown in FIG. 4(C), through the intrafilament meshes 18 (FIG. 2) of the protector 11 to fill a gap 39 between the protector 11 and casing 22 for the later described object. However, most of the sand 13 is retained in the protector 11 to form a sand pillar 12. When the sand 13 is filled up to that part of the protector 11 which is still positioned above the ground 35, then the upper end 11a of the protector 11 is taken off the corresponding outlet 29a of the sand hopper 29 and received in the casing 22 in a folded state.

The upper end 22a of the casing 22 is closed with the sealing cover 36, which is tightened by a latching device 40. Thereafter compressed air is introduced into the casing 22 through an inlet pipe 41 disposed at the upper end of the casing 22. The casing 22 is pulled out of the ground 35 by a drive means 33 (FIG. 8), while being vibrated by the vibro-hammer 24. During the withdrawal of the casing 22, compressed air acts to push the sand drain 10 downward so as to prevent it from being lifted by soft soil. Further, the sand 13 passing through the intrafilament meshes 18 of the protector 11 into the gap 39 between the protector 11 and casing 22 prevents the protector 11 from being brought into contact with the casing 22, namely, acts as a sort oflubricant, thereby saving the protector 11 from damage caused by friction heat resulting from said contact and enabling a sand drain 10 to be firmly set underground without being pulled out together with the casing 22. The application of compressed air and the presence of the sand 13 in the gap 39 jointly help the casing 22 alone to be removed from the ground 35 without causing the protector 11 to be pulled up together. Thus is constructed, as shown in FIG. 4(D), the sand drain 10 having the protector 11 filled with the sand 13. On the sand drain 10 is mounted a load 42.

According to the sand drain-constructing method of this invention, the vibration to which the casing 22 is directly subjected when it is driven into ground and the vibration indirectly imparted to the casing 22 when the protector 11 inserted into the casing 22 is filled with sand 13 cause the water in the soft ground 35 to be gradually collected first to the periphery of the casing 22. When the casing 22 is pulled out, a greater amount of water in the soft ground is gathered around the periphery of the casing 22 due to vibration being imparted thereto. Upon the withdrawal of the casing 22, the collected water is immediately shifted to the pe riphery of the protector 11 filled with sand 13. When the casing 22 is fully pulled out of the ground, compressed air previously introduced into the casing 22 is ejected to the ground surface through the peripheral wall of the protector 11 constituting the sand drain 10, thereby displaying the particularly prominent effect of causing the water already gathered around the periphcry of the protector 11 to push out of the ground at the same time.

According to the sand drain-constructing method of this invention, a plurality of casings arranged in a regular polygonal form are pushed into the ground while being vibrated by the vibro-hammer 24. As easily un' derstood, therefore, the above-mentioned water flows out substantially equally to the ground surface throughout the sand drains.

When the sand drain 10 is constructed in the soft ground 35 as shown in FIG. 4(D), clay particles enter the sand pillar 12 through the intrafilament meshes 18 of the protector 11 together with the water in the soft ground. It is true that clay particles are retained among the sand drains 13 of the sand pillar 12 in increasing amounts with time, slightly reducing the waterdraining capacity of the sand drain 10. As compared with the prior art sand drain, however, wherein the fine meshes of the protector cause a water-impermeable film of clay to be formed on the outer wall of the protector and in consequence obstruct in a relatively short time the passage of the water on the soft ground into the sand pillar in the protector, the sand drain of this invention has a far smaller decrease with time in the waterdraining capacity, because the meshes 18 of the protector 11 are less likely to be clogged. During the normal use of the sand drain of this invention, the above-mentioned clogging can be practically overlooked.

Placement of a load on soft ground generally gives rise to complicated displacements in the various parts of the ground. With the prior art sand drain, therefore, the sand pillar has a very large diameter (generally more than 40 cm) so as to be saved from distortion resulting from the displacement of ground strata and further the protector is prepared from material of very great tensile strength. In contrast, the sand drain 10 of this invention is not made sufficiently rigid like that of the prior art to maintain an upright position against any displacement of ground strata, but, as illustrated in FIG. 4(E), is rendered freely flexible to meet a shearing force while maintaining the continuity of structure, thereby being saved from breakage against any shearing force and always retaining a fixed diameter, whatever position the sand drain 10 may take.

Further, the protector 11 of this invention is formed of fabric 15 bearing rough meshes 18 and is more pliable than the fine meshed protector of the prior art sand drain and assists the flexibility of the sand drain 10.

With the depth of the ground 35 to which the sand drain 10 is driven designated as L and the diameter of the sand drain 10 as D, it has been experimentally proved that if the ratio of L to D exceeds about 40, the sand drain 10 will be fully flexible. Since the sand drain is generally driven to a ground depth of about 10 meters, the diameter D of the sand drain 10 is about 25 cm at maximum. Depending on the condition of the soft ground 35, however, the sand drain 10 is practically chosen to have a diameter D ranging between and 25 cm. The sand drain of this invention has the abovementioned small diameter as against the over 40 cm diameter of the prior art sand drain, enabling a plurality of sand drains 10 to be driven into ground at the same time with a shorter construction period than has been possible in the past.

If Barrons theory is applied to the method of constructing a sand drain underground, time required for the settlement of ground due to consolidation will be proportional to the square of the effective diameter of the influence circle of a sand drain. It is known, therefore, that a dense underground construction of small diameter sand drains has a greater economic merit than the sparse underground arrangement of large diameter sand drains.

Since the load capacity of the sand pillar 12 per unit peripheral length of the protector 11 is proportional to the diameter D of the protector 11, a small protector diameter offers the advantage of reducing the load capacity of the sand pillar 12.

There will now be described by reference to FIG. 9 the method of constructing the sand drain 10 using the protector ll of FIGS. 1 to 4 in soft ground under sea or fresh water. The pile driver 23 is firmly erected by a leader 43 alongside a piling ship 45 afloat on the water 44. A vibrohammer 24 has its height adjusted by being vertically moved along the leader 43 with the aid of a wire 34. From the lower end of the vibro-hammer 24 is suspended a power transmitting rod 46 longer than the water depth L. The rod 46 is inserted into a mount 47. The supporting member 47 is provided with a fixing means 48, which, when tightened, firmly fixes the mount 47 to the rod 46. The fixing means 48 enables the position of the supporting member 47 on the rod 46 to be freely adjusted by vertically moving the supporting member 47 along the rod 46. The casings 22 are securely suspended from the supporting member 47 so as to be arranged in a regular polygonal form. A sand hopper 29 is fixed to the leader 43 such that the outlets 29a of the sand hopper 29 are brought right above the upper ends 22a of corresponding casings 22.

Arms 25, guiding members 26 and guide plate 31 shown in FIG. 9 correspond to those of the pile driver of FIGS. 5 to 8.

There will now be described by reference to FIG. 9 the method of constructing sand drains in the soft ground 35 under water 44. First, the rod 46, together with the vibro-hammer 24, is lifted to a proper height, and the casings 22 are fitted to the supporting member 47 so as to be arranged in a regular polygonal form. A protector 11 shown in FIGS. 1 to 4 is inserted into each casing 22, and thereafter the rod 46 is lifted together with the vibro-hammer 24. The upper ends 22a of the casings 22 are connected to the outlets 29a of the sand hopper 29 so as to fill sand 13 into the protectors 11 already inserted into the casings 22. The vibro-hammer 24 is operated and the casings 22 are driven into the soft ground 35 under the water 44 by the vibration of the vibro-hammer 24 with the aid of the rod 46. Where the upper ends 22a of the casings 22 are still positioned high above the surface 35a of the soft ground 35 when the vibro-hammer 24 is brought down to the lowest position near the upper surface of the sand hopper 29, then the fixing means 48 is loosened to lift the rod 46. With the fixing means 48 tightened at a proper height, the vibro-hammer 24 is again operated to drive the casings 22 into the soft ground 35. Though the water 44 is present on the soft ground 35, the above-mentioned process enables the casings 22 to be driven into the soft ground 35 until the upper ends 22a of the casings 22 are brought close to the surface 35a of the soft ground 35.

The driving of the casings 22 by the pile driver 23 of FIG. 9 can generally be effected to the prescribed ground depth without adjusting the relative positions of the rod 46 and supporting member 47 in the interim or otherwise carrying out said adjustment only once. Where required, however, adjustment of said relative positions may be repeated any number of times. Upon completion of the driving of the casings 22 into the soft ground 35, compressed air is introduced, as previously mentioned, into the casings 22. The casings 22 alone are pulled out by the vibration of the vibro-hammer 24 as well as by the lubricating action of sand 13 (FIG. 4(D)) present in the gap 39 between the casings 22 and the protectors 11 inserted thereinto, thereby constructing the same type of sand drains as shown in FIG. 4(D).

Where casings are driven into soft ground under water by the method and apparatus of the prior art, the upper ends of the casings cannot be submerged underwater, making it necessary to lengthen the casings to an extent at least equal to water depth and consequently giving rise to economic disadvantage. If the protector of this invention should be inserted in such elongate casing, the protector would have to be made long enough to match said extended casing. Otherwise, where there was used a protector long enough to match the ground depth to which the casing was driven, then the protector, after filled with sand, would have to be let fall through the above-mentioned elongate casing. The former case would result on economic disadvantage, while the latter case would cause the protector to be stopped during its fall through the casing, probably failing to reach the bottom of the casing.

In contrast, the method of this invention fully eliminates the drawbacks accompanying the prior art process of constructing a sand drain.

What we claim is:

l. A method for constructing flexible sand drains in soft ground which includes the steps of: arranging in a regular polygonal form a plurality of casings having a sufficient large inner diameter to receive protectors comprising monofilament woven fabric having sufficiently large meshes to permit the passage of part of the sand filled in said protector, and provided at the top with a freely operable sealing cover; driving said casings into said soft ground at the same time by a vibrohammer mounted on a pile driver; connecting the upper ends of said protectors to the outlets of a hopper mounted on said pile driver after said protectors are inserted into said casings and filling sand in said protectors by the vibration of said vibro-hammer to form sand pillars; introducing compressed air into said casings with the respective sealing covers closed; pulling said casings alone out of said soft ground while being vibrated by said vibro-hammer with the lubricating action of the sand passing through a gap between said casings and protectors, thereby firmly setting in said soft soft ground according to claim 2 wherein said casings are driven into soft ground under water by a power transmitting rod provided between said vibro-hammer and casings until the upper ends of said casings are brought close to the surface of said under water ground. 

1. A method for constructing flexible sand drains in soft ground which includes the steps of: arranging in a regular polygonal form a plurality of casings having a sufficient large inner diameter to receive protectors comprising monofilament woven fabric having sufficiently large meshes to peRmit the passage of part of the sand filled in said protector, and provided at the top with a freely operable sealing cover; driving said casings into said soft ground at the same time by a vibro-hammer mounted on a pile driver; connecting the upper ends of said protectors to the outlets of a hopper mounted on said pile driver after said protectors are inserted into said casings and filling sand in said protectors by the vibration of said vibro-hammer to form sand pillars; introducing compressed air into said casings with the respective sealing covers closed; pulling said casings alone out of said soft ground while being vibrated by said vibro-hammer with the lubricating action of the sand passing through a gap between said casings and protectors, thereby firmly setting in said soft ground sand drains comprising said protectors and sand pillars contained therein.
 2. The method of constructing flexible sand drains according to claim 1 wherein, prior to the driving of said casings into said soft ground, said protectors are inserted into said casings and sand is placed in said protectors.
 3. The method of constructing flexible sand drains in soft ground according to claim 2 wherein said casings are driven into soft ground under water by a power transmitting rod provided between said vibro-hammer and casings until the upper ends of said casings are brought close to the surface of said under water ground. 